Correlation system with recirculating reference signal for increasing total correlation delay

ABSTRACT

A correlation system includes a delay line correlator comprising two tapped delay lines where an input signal and a reference signal are applied to the lines, respectively, such that the signals scan past each other at a relative velocity with respect to each other for obtaining the correlation function therebetween. Means are included in the system for repetitively inserting the reference signal into the correlator for increasing the total correlation delay time thereof.

United States Patent Cook Nov. 20, 1973 CORRELATION SYSTEM WITH3,419,867 12/1968 Pifer 343 100 CL RECIRCULATING REFERENCE SIGNAL 3 228Z 13;; {9 3: i

2 3 FOR INCREASING TOTAL CORRELATION 5 1 9/1970 g gj DELAY [75]lnventor: Charles E. Cook, Carlisle, Mass. OTHER PUBLICATIONS Tien:Parametric Amplification and Frequency Mixing [73] Asslgnee: Sperry RandCorporatlon, New in Propagating Circuits, Journal of Applied Physics,

York, v01. 29, No. 9, Sept. 1958.

[22] Filed: Sept. 20, 1971 Prima ExaminerFe1ix D. Gruber [21] Appl'181398 Attorn Zy-S. C. Yeaton [52] US. Cl 235/181, 330/55, 343/17.l, 57ABSTRACT [51 1 Int Cl ggaf g A correlation system includes a delay linecorrelator 581 Field of Search 235/181; 343/111, i delay Y 3 511%343/100 330/5 307/88 3 nal and a reference s1gnal are appl1ed to thelines, re-

spectively, such that the signals scan past each other 56] ReferencesCited at a relatIve veloc1ty w1tl'1 respect to each other for ob- TEDSTATES PATENTS taming the correlation funct1on therebetween. Means UNIare included in the system for repetitively inserting the 3,463,9ll8/1969 Dupraz et al. 235/181 reference ignal into the correlator forincreasing the 3,665,413 5/1972 Walsh et a1. 235/181 X total con-elationdelay time thereof. 3,599,209 8/1971 Goodrich 235/181 X 3,592,182 7/1971Menard 235/181 X 9 Claims, 5 Drawing Figures 54 i 51 GATE TO DETECTORFROM SYSTEM 8 VIDEO AMPLIFIER IFAMPLIFIER i ANDDISPLAY GATE DELAY DEVICEk S (f TD T s NO 1 s5 52 I 1 s(1+[2n+11T 1 DELAY TIMING (2 n :1.) T D SY S T E M SYNC PROGRAMMABLE 40 O R E F E R E N CE 51 G K s GENERNAATLORCONTROL 41 g, 50

E L EC T R0 N C SW 1 TC H GA TE 3 Tl M E 5 (1 INVERSION T o TER L E A DI N G E DGE T RIG G E R G E N PAIENIEnunvzo ms 3.774.019 sum 1 u 4 DELAYLINE OS2(T) o DELAY LINE v 8 ho") h (2f) s n) 5 16 11 *QQCL c h (2f)PRIOR ART .Z'A/VE/VTO/P CHARLES 5. 600K PAIENTED IIUY 2 0 I973 3,774,0183:555 ELI 4 SIGN AL SPATIAL DISTRIBUTION IN DELAY MEDIUM SIGNAL NPUTS2(I)=S I I I I I I 20 I I I I I I 21 I I I t I T 02 02 I I |Q DE LAY MED I UMD| I I I I REF. SIGNAL S(T +T ATTORNEY FIG.3.

CORRELATION SYSTEM WITH RECIRCULATING REFERENCE SIGNAL FOR INCREASINGTOTAL CORRELATION DELAY BACKGROUND OF THE INVENTION 1. Field of theInvention The invention relates to systems for providing the correlationfunction between an input signal and a reference signal particularlywith regard to target ranging systems.

2. Description of the Prior Art Target ranging systems such as, forexample, radar systems are known in which a signal is transmitted andreflected from a target, the total transit time of the signal from thetransmitter to the receiver providing a measure of the range to thetarget. Correlation devices have been utilized in such systems toenhance the signal to noise ratio of the target return signals. In suchcorrelation devices a replica of the transmitted signal is stored in atemporally or spatially stationary manner and the input signal from thereceiver is correlated therewith over the entire range of interest ofthe system. In such systems, it is often difficult to alter the storedreference so as to accommodate expected distortions in the return, i.e.,from doppler shift, or to enable changing the transmitted signal for,for example, coding or covert transmission purposes.

A deltic correlator may also be utilized in such systems. A delticcorrelator is a digital device of the type wherein the reference signalis stored in a temporally stationary manner. Between sampling intervalsof the input signal, the bits of the reference and the bits of the inputsignal are circulated in the same direction past a circuit that providesthe bit by bit comparison therebetween required in obtaining thecorrelation function. After the circulation the bits of the referenceare returned to their initial position and the input signal is thenshifted by one bit position. Thus, it will be appreciated that thedeltic correlator requires complex clocking and timing circuits so as togenerate the correlation function and provide a useful display thereof.

Correlation devices are also known wherein an input signal and areference signal are scanned relative to each other where the twosignals propagate in opposite directions through the device. Absent thepresent invention, these devices are not particularly well suited totarget ranging systems of the type described since the measurable rangeinterval would be limited by the delay time through the device. Delaytimes of available delay devices are of such magnitude so as to providean extremely limited operable range for such systems.

Microwave acoustic delay line correlators of this type are known thatpossess the advantages of small size and relative simplicity. Thesedevices, of course, suffer from the disadvantages herein discussed. Sucha device is described in Vol. 16, No. 12 of Applied Physics Letters ofJune 15, 1970, on page 494 in the C. F. Quate and R. B. Thompson letter,Convolution and Correlation in Realtime with Non-Linear Acoustics." Forsuch a device, correlation is performed by propagating the two signalsin the same direction through the device at different velocities withrespect to each other. Since the correlation is performed effectivelybetween dissimilar signals, a maximum correlation output does notresult. Furthermore, when the two signals are propagated through thedevice in opposite directions at the same speed with respect to themedium, the convolution function therebetween is obtained providing asubstantially smaller output than that produced when correlation isperformed.

SUMMARY OF THE INVENTION The present invention obviates the hereinabovedescribed disadvantages of the prior art devices by providing acorrelator system wherein the input signal and the reference signal arescanned past each other at a relative velocity with respect to eachother thereby obtaining the correlation function therebetween. Means areincluded in the system for repetitively inserting the reference signalinto the correlation device to increase the total correlation delay timethereof, thereby providing an arbitrarily long operable range for atarget ranging system in which the device may be utilized.

Where required, means are included to invert the time scale of one ofthe two signals being correlated such that propagation in oppositedirections, respectively, of the two signals provides the correlationfunction rather than the convolution function as desired.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic block diagram ofa delay line correlator of a type that may be used in the correlationsystem of the invention;

FIG. 2 is a schematic representation of temporal and spatial waveformdistributions useful in explaining the operation of FIG. 1;

FIG. 3 is a schematic representation of temporal and spatial waveformdistributions with respect to the delay medium of FIG. 1 useful inexplaining the application of the invention to target ranging systems;

FIG. 4 is a schematic block diagram of a portion of a target rangingsystem and embodies a correlation system incorporating the concepts ofthe present invention; and

FIG. 5 is a waveform timing diagram illustrating waveforms at variouspoints of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a delayline correlator 10 of a type that may be used in the correlation systemof the invention is illustrated. The delay line correlator 10 iscomprised of tapped delay lines 11 and 12 having signals 8 (1) and 8 (1)applied to the inputs thereof, re-

spectively. Each tap of the delay line 11 is applied as an input to amultiplier 13, the other input of which is provided by the correspondingtap of the delay line 12. The output of the multipliers 13 are appliedas inputs to a summing network 14 that provides the output of the deviceh,,(2t) on a lead 15.

It will be appreciated that the arrangement of the components of FIG. 1is a conventional configuration for providing the convolution functionor the correlation function between the signals 8 (1) and S (t) inaccordance with the manner in which the signals are applied to the delaylines 11 and 12.'The delay line correlator 10 may be considered as adiscrete component model of the microwave acoustic correlation andconvolution device of the said Quate and Thompson letter in which thetaps of the delay lines 11 and 12 are very closely spaced, i.e., A rl/highest frequency component of the signal. When the signals S (t) andS,(t) are applied at opposite ends of the Quate and Thompson device sothat the respective acoustic waves are propagating in oppositedirections in the medium and with velocities of equal magnitude, thenon-linear interaction between the waves within the medium results in anoutput: h,,(2t) I S,(-r)S2(2t-r) d1- which represents the convolution ofS, (t) and 52(1), except for the scale factor of 2 with regard to theparameter t. This scale factor has the effect of halving the timedimension of the output signal. It is thus appreciated that with thesignals S (t) and 8 (1) applied to opposite ends of the respective delaylines 11 and 12 of FIG. 1, the output h,,(2t) on the lead 15 will be theconvolution of the input signals. FIG. 1 also illustrates curves l6 and17 representative of the output of a device in which one of the signalsis stationary and the output of a device such as the delay linecorrelator 10, respectively. The time compression factor of 1/2 of thewaveform 17 with respect to the waveform 16 is depicted for clarity.

It is conventional practice with devices such as illustrated in FIG. I,or with microwave acoustic devices such as the aforesaid Quate andThompson device, to provide the correlation function between the twosignals S,(r) and 8 (2) by applying the signals to the same end of thedevice such that they travel in the same direction through the mediumbut with different propagation velocities with respect to each other toprovide the relative scanning therebetween required for correlation. Indevices of the microwave acoustics variety, one of the signals may bepropagated as a longitudinal wave and the other as a shear wave therebyproviding the relative velocities therebetween. Because of the differentvelocities of the two propagating waves, the correlation is effectivelybetween dissimilar signals and thus does not provide a maximumcorrelation output. This is undesirable in signal processing systemswhere maximum signal enhancement is required for efficacious operation.

Devices of the type discussed conventionally provide the convolution ofthe signals S (t) and S (t) by apply ing these signals to opposite endsof the device such that the resulting waves propagate through the mediumin opposite directions and with equal velocities thus providing theconvolution function as previously discussed. As is well known, theconvolution between two signals normally is of a smaller magnitude thanthe correlation therebetween and thus is not as useful a function ascorrelation for signal processing applications for the reasons givenabove.

Devices of the type described above may be utilized to provide thecorrelation function between the two signals S,(t) and S (t) by applyingthe signals to opposite ends of the device such that the resulting wavepropagates at the same velocity but in opposite directions relative toeach other and inverting the time scale of one of the two signals.

Referring now to FIG. 2, schematic representations of temporal andspatial distributions of the waveforms S,(t) and S (t) are illustratedwith respect to the delay medium of the aforedescribed devices. Thesignal S,(t) indicated at 20 is applied to one end of the delay medium.The waveform 20 is illustrated as a stylized frequency modulated signal.The required time scale inversion is effected with respect to the signalS (t) by utilizing S (t) S,(-t) as illustrated at 21. The waveform 21 isthen applied to the other end of the delay medium. Within the delaymedium the waveforms 20 and 21 now have identical spatial distributionsso that 5 (1) may be considered as a reference signal identical withinthe medium to the input signal S,(t) and scanning past it. Thus, forthis situation, the non-linear interaction of the two signals within themedium provides a correlation output [1,,(2!) I S (r)S2(2t+r) dr whichis desired for signal processing applications for the reasons discussed.

It will be appreciated that in many applications where even functionsignals such as pulse trains are utilized, the correlation andconvolution functions provide identical results except for the sign ofthe time axis. Generally, however, for radar or communication systems,the complex signals utilized do not exhibit this type of symmetry sothat the convolution and correlation functions do not result in similaroutput signals. For these types of waveforms, the correlation output isthe more desirable of the two to provide maximum signal to noise ratios.Complex signals of this type are described in the textbook, RadarSignals by C. E. Cook and M. Bernfeld published in 1967 by the AcademicPress.

It will be appreciated with regard to FIG. 2 that when the two signalsS,(t) and S (t) are applied to the same end of the delay medium withouttime inversion so as to effect correlation therebetween, as previouslydescribed, and since one of the waves travels at a faster velocity thanthe other wave, the two signals are not spatially commensurate withinthe medium and the resulting correlation is effectively betweendissimilar signals thus not providing a maximum correlation output aspreviously discussed. It may further be appreciated that when the twosignals are applied to the opposite end of the medium without timeinversion for providing the convolution therebetween as previouslydiscussed, the two waveforms within the medium are again not spatiallycommensurate unless the waveforms are special symmetrical signals.

It is desirable to utilize correlation devices in target detectionsystems such as radar for enhancing the signal to noise ratio of thereturns from the targets. Devices of the type described above provide alimited operable range since the total range delay over whichcorrelation may be performed is limited by the total delay of thedevice. Particularly, the microwave acoustic correlator described aboveis similarly limited but can provide the same type of processing as thediscrete component correlator 10 of FIG. 1 but with a much smallerstructure. In accordance with the principal concept of the presentinvention, this limitation is overcome by providing means forsuccessively re-inserting the reference signal at accurately timedintervals to permit the device to correlate signals over a substantiallylonger range delay interval than the intrinsic delay of the structure.

In order to utilize the above-described correlation devices as rangeprocessors, the intrinsic delay of the device should be sufficientlylong so that more than one serially time-distributed range bin maysimultaneously be contained in the delay medium. This is required sothat every target return will be scanned by a reference signal.

Referring to FIG. 3, a schematic representation of temporal and spatialwaveform distributions with respect to the delay medium of the deviceshereinabove described, is illustrated. Three consecutively arrivingtarget return signals 30, 31 and 32 are depicted where the delay time ofthe medium is sufficiently long to simultaneously accommodate returns 30and 31, which length is designated as T T for reasons to be discussed.The reference signal 33 is delayed in a manner to be described so as toenter the line when the return 30 is about to exit therefrom so thatthere isnt any portion of the delay medium that is not used for inputsignal correlation. As depicted, the reference 33 scans past the threetarget returns 30, 31 and 32 at the time intersections indicated at 33,33" and 33", respectively, to effect correlation therebetween.

As previously described, because the input signal and the referencesignal are traveling through the medium in opposite directions at thesame velocity magnitudes, the correlation output signal is compressed intime by a factor of 2. Because of this, if the device delay time T issubstantially greater than the signal duration T then the device iscapable of processing the signals that are contained within a rangeinterval 2T Thus, in practicing the invention the reference signalshould be re-inserted into the delay medium preferably at every otherinterval T so that ambiguities are not generated by the same targetreturn correlating with two consecutively inserted reference signals.Thus it is appreciated that by repetitively re-inserting the referencesignal at every other interval T the contiguous range intervals ofduration 2T may be processed by the device in accordance with theinvention. Because of the time scale change factor, all signalscontained in an interval 2T at the device input, will occupy an intervalof 1",, at the device output. Thus if by means of re-inserting thereference signal, signals occupying an interval ZNT are processed, thenthe processed output will comprise contiguous intervals of duration Talternately containing processed signals and no signal, in a manner tobe further explained. It will be appreciated that although the delaytime for the medium must be at least long enough to simultaneouslyaccommodate two contiguous range bins, the line may be arbitrarily longand it is designated as having a length of T T, where 2T is the timeduration between reinsertions of the reference and T, is the length ofthe input signal as well as the length of the reference signal.

Referring now to FIGS. 4 and 5, a schematic block diagram of thecorrelation processing system of a target ranging system incorporatingthe concepts of the present invention and a waveform timing diagram ofsignals at various points of FIG. 4, are illustrated, respectively. Asystem sync pulse from the target ranging system is applied to trigger aprogrammable reference signal generator 40. The signal generator 40 isalso responsive to a program control signal on a lead 41 for controllingthe type of signal provided thereby. The programmable reference signalgenerator 40 is of a conventional type well known in the radar andcommunications art for providing a finite duration signal S(t). Thesignal S(t) may be any one of a wide variety of known signals such asswept FM phase codes and pulse trains. The signal generator 40 may alsobe of a type described in the said Cook and Bernfeld textbook thatprovides a pulse compression signal having a signal timebandwidthproduct or compression ratio of T,,W,, where W, is the signal bandwidth.When a signal of this type is provided, the number of independent rangecells that may be examined at the delay line correlator output is givenby 2T T, W,/T, or 2T W,. For example, if the signal time-bandwidthproduct is 100 and the total delay of the device is 6T,, so that T ST,,,then each output signal interval will contain information from 1,000range cells, each of width l/W,.

The programmable reference signal generator 40 may alternatively be of adigital design of the type described in US. Pat. application Ser. No.1,090 filed Jan. 7, 1970, A Digital Waveform Generator, by A. W. Crookeand M. E. Hanna, Jr. now US. Pat. No. 3,633,017 issued Jan. 4, 1972 andassigned to the assignee of the present invention.

The output of the signal generator 40 is selectively applied to either alead 42 or a lead 43 via an electronic switch 44 in response to thesystem sync signal. The system sync signal conditions the electronicswitch 44 so that the first occurrence of the signal S(t) is applied tothe lead 42 and subsequent occurrences thereof are applied to the lead43 until the appearance of the next system sync pulse.

The lead 42 provides the input to a time inversion circuit 45 whoseoutput is the time inverted finite duration signal S(t). When S(t) is asignal with linear or odd symmetric frequency modulation, then S(t) maybe derived from S(t) in a conventional manner by the well knownside-band inversion technique disclosed in US. Pat. No. 3,400,396 issuedSept. 3, 1968, Pulse Stretching And Compression Radar System, by C. E.Cook and C. E. Brockner and assigned to the assignee of the presentinvention.

Other classes of waveforms, with for example complex phase modulationcharacteristics of the type described in the said Cook and Bernfeldtextbook may be synthesized using digital signal generation methodswhich are readily adaptable to digital storage and reverse readout toobtain the desired time inversion. Accordingly, the device disclosed insaid patent application Ser. No. 1,090 is an example of such digitalapparatus and would in fact encompass the programmable reference signalgenerator 40 as well as the time inversion circuit 45.

The output S(t) from the time inversion circuit 45 is sent to the radartransmitter (not shown) to be radiated in a conventional manner forreflection from targets within the range of the system. The signal S(t)from the time inversion circuit 45 is also applied to a leading edgetigger generator 46 which provides a pulse upon the occurrence of theleading edge of S(t) in a conventional manner. It will be appreciatedthat the time inversion circuit 45 may introduce some delay in providing its output S(t) from its input S(t) as illustrated in FIG. 5. It istherefore appreciated that the system timing is referenced to the outputof the leading edge trigger generator 46, i.e., to the leading edge ofthe transmitted signal S(t), in a manner to be described.

The output of the leading edge trigger generator 46 is applied to adelay timing circuit 47 which is triggered by the output of the leadingedge trigger generator 46 to provide a train of pulses at intervals of(Zn +1 )T These pulses therefore occur at intervals T ,3T 5T,,, etc. Thedelay timing circuit 47 is of conventional design and may beinstrumented as a binary counter that is triggered by the leading edgetrigger generator 46 and whose overflow signals provide the designatedpulses to the programmable reference signal generator 40.

The output of the delay timing circuit 47 is applied as an input to theprogrammable reference signal generator 40 wherein for each pulsereceived'the signal generator 40 provides the signal S(t) to theelectronic switch 44. It is therefore appreciated that the electronicswitch 44 provides the signal S(t) on the lead 42 to the time inversioncircuit 45 in response to the systems sync signal and thereafterprovides the signal S(-t) on the lead 43 in response to the pulses fromthe delay timing circuit 47.

The lead 43 signal is applied as an input to a gate 50 which alsoreceives a control signal P; as illustrated in FIG. as a second inputthereto. The outputs of the gate 50 are therefore the signals S(-z)triggered by the delay timing circuit 47 which propagate through thegate 50 in the presence of the pulse P This gate 50 output may bedesignated as S(-t [2n 1] T,,) as indicated by the legend.

The output of the gate 50 is applied as the reference input signal to adelay correlation device SI which may be of the type discussed abovewith regard to FIG. 1 or may be of the microwave acoustic variety of thetype discussed with respect to the said Quate and Thompson letter. Thecorrelator 51 may also be instrumented in digital form with shiftregister delay lines in a manner well understood in the art.

The target returns of the S(z) signal that was transmitted as previouslydescribed are received and applied to a lead 52 from the system if.amplifier. The target returns of the finite duration S(r) signal arereceived at arbitrary arrival times within the predetermined range timeinterval of the target ranging system. The duration of the finiteduration signal is small compared to the predetermined range timeinterval as seen in FIG. 5. The received signal may be designated asKS(t +7) noise where K is a range attenuation factor and r is the delayto a target from which the signal has been reflected. The target returnson the lead 52 are applied as an input to a gate 53 which receives a Fcontrol signal as illustrated in FIG. 5 at a second input thereto. Thecontrol signal P is initiated in response to the output of the leadingedge trigger generator 46 and therefore renders the gate 53 conductiveat the leading edge of the radar transmission in anticipation of targetreturns.

The output of the gate 53 is applied as the input signal to the delayline correlator 5H for correlation therein with the reference signalfrom the gate 50 in the manner previously described.

The output from the correlation device 51 is applied to a gate 54 whichalso receives control signal I as illustrated in FIG. 5 at the secondinput thereto. The output of the gate 54 provides the signal for thedetector, video amplifier and display portion of the target rangingsystem not shown for simplicity.

In operation, a system sync pulse sets the electronic switch 44 toprovide its output on the lead 42. The system sync pulse also triggersthe reference signal generator 40 to provide the signal S(t) to the timeinversion circuit 45 which, in turn, provides the inverted signal S(z)to the transmitter. The electronic switch 441 is configured tothereafter connect the signal S(t) to the lead 43 for application to thegate 50. The leading edge of the transmission causes the leading edgetrigger generator 46 to provide a pulse to the delay timing circuit 47that in turn provides pulses at the intervals T 3T,,, 51],, etc. Thesetriggers from the delay timing circuit 47 cause the programmablereference signal generator 40 to provide the reference signal S(t)through the gate 50 as the correlation reference for the delay linecorrelating device 51. It is appreciated that since the reference S(-t)is time inverted by the time inversion circuit 45 before transmission,the reference signal from the gate will be the time inverse of the inputsignal from the gate 53 therefore performing correlation as describedabove.

It will be appreciated that the return from a target at zero range willreach the end of the delay line into which it wias inserted at theinterval T after the leading edge trigger from the circuit 46. Since thefirst delay timing pulse from the circuit 47 sends the reference signalinto the device 51 at this time, full utilization of the delay medium isachieved for input correlation as previously described. Since the totaldelay for the medium is T T and the reference signal is reinsertedtherein at the intervals (2n 1) T there isnt any loss of correlatedsignal response when the reference signal is at the point at maximumdelay before exiting the medium. The timing of P then eliminates fromthe output display any partially correlated signal resulting from thereference signal being partially within the medium such as when it isjust entering or just exiting the medium. The returns corresponding tothese eliminated partially correlated signals are then fully correlatedby the next re-insertion of the reference at which time the P controlsignal passes the processed signals through the gate 54 to the output.

Because of the 2 to 1 time compression of the delay correlator 511 aspreviously discussed, the output therefrom is comprised of alternateintervals of duration T,, of processed signal and no-signal asillustrated in FIG. 5 and as transmitted through the gate 54 by thecontrol signal P It is appreciated that during the intervals ofno-signal other operations such as frequency analysis may be performed.It is furthermore appreciated that because of the time-scale change, itis necessary for any amplifying or processing circuits that receivetheir input from the gate 54 to have essentially twice the bandwidth ofthe original input signal S(t).

After sufiicient re-insertions of the reference signal from the gate 50to cover the range of interest for the system, the gates 50, 53 and 54are closed a short time prior to the next system sync pulse to allow allsignals to settle out.

It will be appreciated that the delay line correlator 51 may be designedto impart pulse compression to the target return signals as discussedabove, and the programmable reference signal generator designed toimpart the complementary pulse expansion to the transmitted signals inthe manner discussed in the said Cook and Bernfeld textbook. It will beappreciated that the timing and gating pulses illustrated in FIG. 5 arereadily obtainable from conventionally designed circuits that are notshown for simplicity.

Since the processor disclosed hereinabove does not depend for itsoperation of any specific signal format or modulation, the referencesignal applied to the correlator 5B. may be altered as a function ofexpected variations in the parameters of the received signal as afunction of range or frequency. For example, the reference signal may beidentical to the transmitted signal except for a doppler frequencyshift. A number of all-range processors may be operated in parallel as ameans of identifying a specific doppler shift, the reference signals foreach processor differing only in their doppler shift parameters. Thereference signal may be varied as a function of range to reflect variousdetection conditions. As an example, if the transmission is arectangular envelope linear FM signal, the replica signal for nearranges would also be a rectangular envelope, full duration signal toachieve optimum processing gain in clutter or reverberation which tendsto predominate at near ranges. At far ranges, the envelope of the signalmay be tapered and truncated to achieve a processor output with lowrange sidelobes so as not to obscure small signals that are above thenoise level but which are smaller in amplitude than a larger signal atapproximately the same range. A similar type of reference signalvariation as a function of time may be used in a communicationapplication that employs signal coding within a frame time. By means ofthe changes in the signal structure, different frame times may beallocated to different users.

It will be appreciated that an alternative arrangement for the apparatusof FIG. 4 may be realized by connecting the lead 42 directly to theleading edge trigger generator 46 and to the transmitter and insertingthe time inversion circuit 45 in the lead 43. With this embodiment, theprogrammable reference signal generator 40 may be instrumented toprovide S(t) which will be transmitted and the time inversion circuit 45will provide S(t) through the gate 50 to the delay correlator device 51.

While the invention has been described in its preferred embodiment, itis to be understood that the words which have been used are words ofdescription rather than limitation and that changes may be made withinthe purview of the appneded claims without departing from the true scopeand spirit of the invention in its broader aspects.

I claim:

1. A correlator system for correlating a finite duration input signalwith a corresponding finite duration reference signal, said input signalhaving an arbitrary arrival time in a predetermined time interval and asmall duration compared to said time interval, comprising correlatormeans having a correlation delay time substantially less than saidpredetermined time interval and responsive to said signals forpropagating both said signals through said correlator means in oppositedirections with respect to each other for scanning said signals pasteach other at a relative velocity with respect to each other to obtainthe correlation function therebetween, and

means for repetitively inserting said finite duration reference signalinto said correlator means for increasing the total correlation delaytime of said correlator means to provide a correlation peak with respectto said input signal in accordance with said arbitrary arrival time.

2. The system of claim 1 in which said correlator means includes firstand second delay line means for effectively propagating both said inputsignal and said reference signal, respectively, past each other.

3. The system of claim 2 in which said means for repetitively insertingincludes means for inserting said reference signal into said correlatormeans at intervals related to twice the delay time of said delay linemeans.

4. The system of claim 1 in which said correlator means comprises amicrowave acoustic correlator.

5. The system of claim 1 further including means for time inverting oneof said signals prior to application to said correlator means.

6. A correlator system for use in a target ranging system having apredetermined range time interval and including a signal source forproviding a first finite duration signal of small duration compared tosaid time interval and means for time inverting said first signal toproviding a second finite duration signal, said target ranging systemtransmitting one of said first and second signals and receiving returnsignals thereof reflected from said targets at arbitrary arrival timesin said time interval, said correlator system comprising correlatormeans having a correlation delay time substantially less than saidpredetermined time interval and responsive to said return signals and tothe other of said first and second signals for propagating both saidsignals through said correlator means in opposite directions withrespect to each other for scanning said signals past each other at arelative velocity with respect to each other to obtain the correlationfunction therebetween, and

means for repetitively inserting said other signal into said correlatormeans for increasing the total correlation delay time of said correlatormeans to provide correlation peaks with respect to said return signalsin accordance with said arbitrary arrive] times.

7. The correlator system of claim 6 in which said correlator meansincludes first and second delay line means for effectively propagatingboth said return signal and said other signal, respectively, past eachother.

8. The correlator system of claim 7 in which said means for repetitivelyinserting includes means for inserting said other signal into saidcorrelator means at intervals related to twice the delay time of saiddelay line means.

9. The correlator system of claim 6 in which said correlator meanscomprises a microwave acoustic correlator.

1. A correlator system for correlating a finite duration input signalwith a corresponding finite duration reference signal, said input signalhaving an arbitrary arrival time in a predetermined time interval and asmall duration compared to said time interval, comprising correlatormeans having a correlation delay time substantially less than saidpredetermined time interval and responsive to said signals forpropagating both said signals through said correlator means in oppositedirections with respect to each other for scanning said signals pasteach other at a relative velocity with respect to each other to obtainthe correlation function therebetween, and means for repetitivelyinserting said finite duration reference signal into said correlatormeans for increasing the total correlation delay time of said correlatormeans to provide a correlation peak with respect to said input signal inaccordance with said arbitrary arrival time.
 2. The system of claim 1 inwhich said correlator means includes first and second delay line meansfor effectively propagating both said input signal and said referencesignal, respectively, past each other.
 3. The system of claim 2 in whichsaid means for repetitively inserting includes means for inserting saidreference signal into said correlator means at intervals related totwice the delay time of said delay line means.
 4. The system of claim 1in which said correlator means comprises a microwave acousticcorrelator.
 5. The system of claim 1 further including means for timeinverting one of said signals prior to application to said correlatormeans.
 6. A correlator system for use in a target ranging system havinga predetermined range time interval and including a signal source forproviding a first finite duration signal of small duration compared tosaid time interval and means for time inverting said first signal toproviding a second finite duration signal, said target ranging systemtransmitting one of said first and second signals and receiving returnsignals thereof reflected from said targets at arbitrary arrival timesin said time interval, said correlator system comprising correlatormeans having a correlation delay time substantially less than saidpredetermined time interval and responsive to said return signals and tothe other of said first and second signals for propagating both saidsignals through said correlator means in opposite directions withrespect to each other for scanning said signals past each other at arelative velocity with respect to each other to obtain the correlationfunction therebetween, and means for repetitively inserting said othersignal into said correlator means for increasing the total correlationdelay time of said correlator means to provide correlation peaks withrespect to said return signals in accordance with said arbitrary arriveltimes.
 7. The correlator system of claim 6 in which said correlatormeans includes first and second delay line means for effectivelypropagating both said return signal and said other signal, respectively,past each other.
 8. The correlator system of claim 7 in which said meansfor repetitively inserting includes means for inserting said othersignal into said correlator means at intervals related to twice thedelay time of said delay line means.
 9. The correlator system of claim 6in which said correlator means comprises a microwave acousticcorrelator.